1. Field of the Invention
This invention relates to light emitting diodes (LED or LEDs) and more particularly to methods for coating LEDs with a conversion material and LED packages fabricated using the methods.
2. Description of the Related Art
Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different colors from different LEDs. For example, the light from red, green and blue LEDs, or blue and yellow LEDs can be combined to produce white light.
Light from a single blue emitting LED chip has been converted to white light by surrounding the LED chip with a yellow phosphor, polymer or dye. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc., which comprise blue LEDs surrounded by a yellow phosphor powder; see also U.S. Pat. No. 5,959,316 to Lowery, entitled Multiple Encapsulation of Phosphor-LED Devices.] The surrounding material downconverts the wavelength of some of the LED light, changing its color. For example, a nitride based blue emitting LED chip can be surrounded by a yellow phosphor. Some of the blue emitted light can pass through the phosphor without being changed while the remaining light can be downconverted to yellow. The LED chip emits both blue and yellow light, which combine to produce white light. Another example of LEDs using this approach is disclosed in U.S. Pat. No. 5,813,753 to Vriens et al.
A common type of LED packaging where a phosphor is introduced over an LED is known as a “glob-in-a-cup” method. An LED chip resides at the bottom of a cup-like recession, and a phosphor containing material (e.g. phosphor particles distributed in an encapsulant such as silicone or epoxy) is injected into and fills the cup, surrounding and encapsulating the LED. The encapsulant material is then cured to harden it around the LED. This packaging, however, can result in an LED package having significant variation of the color and hue of emitted light at different viewing angles with respect to the package. This color variation can be caused by a number of factors, including the formation of a non-uniform layer of conversion and encapsulant on the LED surface that emits light. This problem can be made worse in packages where the phosphor containing matrix material extends above the “rim” of the cup in which the LED resides, resulting in a predominance of converted light emitted sideways into high viewing angles (e.g. at 90 degrees from the optic axis). The result is that the white light emitted by the LED package becomes non-uniform and can have bands or patches of light having different colors or intensities.
Another method for packaging or coating LEDs comprises direct coupling of phosphor particles onto the surfaces of the LED using methods such as electrophoretic deposition. This process uses electrostatic charge to attract phosphor particles to the surface of the LED chip that is charged. This “white chip” method can result in improvement of the color uniformity as a function of viewing angle with one reason for this improvement being the source of the converted light and unconverted light being at close to the same point in space. For example, a blue emitting LED covered by a yellow converting material can provide a substantially uniform white light source because the converting material and LED are at close to the same point in space. This, however, is typically a complex and costly method for achieving uniform phosphor coating directly on an LED. The phosphor particles are first suspended in a solvent and allowed to flow to the surface and remain attracted to the LED surface by the charge. The method can be slow, messy and can present inconsistencies due to difficulties in controlling electrostatic charges across many LEDs in a mass production environment.
Existing LED packages can also utilize a relatively large volume of encapsulant which can have a different coefficient of thermal expansion compared to the remaining LED package components, such as the reflector, circuitry substrate, optical elements, etc. This can put stresses on the components through the operating temperature cycle of the LED package that can result in reliability issues such as delamination of encapsulant from reflector or substrate walls, cohesive fracture within the encapsulant itself, and damage to LED chip wire bonds.